Does your plasma TV have a feature called something like "dynamic contrast"? That could be adjusting the overall brightness to darken darker scenes and brighten brighter scenes in order to increase the apparent contrast ratio past the native contrast ratio of the panel. Actually, I think it's more common in LCD TVs. I certainly had to turn it off in my (several year old) Mitsubishi to avoid noticeable shifts in brightness right after a scene change.
9/27/12 at 7:56am